6.5 Grendel Firing Pin Protrusion - Which Pin Goes in Which Bolt

Some 6.5 Grendel bolts need a 6.5 Grendel firing pin. Put that same pin in a different Grendel bolt and you've built yourself a light-strike machine. Nothing on the box tells you which bolt you have.

In our breakdown of 6.5 Grendel bolts, we covered how the Grendel's deeper .136" bolt face throws off the firing pin math, and how Alexander Arms got around it by making their bolt about .010" longer so a standard AR-15 firing pin still works. Not every bolt maker copied that fix. So we took two firing pins and three Grendel bolts and measured what actually happens.

The Pins

A standard mil-spec AR-15 firing pin should be between 3.271" and 3.280". That's the 3.280" maximum with a minus .009" tolerance you'll see quoted on the forums, though the same forums also quote 3.280" plus or minus .001". We couldn't find the actual drawing behind either one, and the Army's maintenance manuals don't list a pin length at all. They only check protrusion, which should tell you something.

The AR-15 firing pin we used for this test was 3.278 inches. The Lantac 6.5 Grendel firing pin selected for this test was measured to be 3.270 inches. We ran both of them in three .136" Type II bolts: a Rexus, a JP Enterprises, and a Craddock Precision HD ARC. We measured protrusion with an iGaging digital electronic depth gauge and a special tip from SOTAR.

The Spec

Firing pin protrusion is how far the tip of the firing pin sticks out past the bolt face when the pin is pushed all the way forward. On an AR-15 it should be between .028" and .036". That window comes from the military's M16 and M4 maintenance manuals, and it's the same one every Grendel builder uses, because nobody we could find has ever published a separate one for the Grendel.

The whole window is .008" wide. A sheet of printer paper is about .004" thick, so the difference between a pin that's too short and a pin that's too long is about two sheets of paper.

What We Measured

Standard AR-15 firing pin (3.278"):

• Rexus: .0305"
• JP Enterprises: .0325"
• Craddock Precision HD ARC: .0410"

Lantac 6.5 Grendel firing pin (3.270"):

• Rexus: .0195"
• JP Enterprises: .0215"
• Craddock Precision HD ARC: .0305"

Three of the six combinations are in the window and three are out. The standard pin in the Craddock is .005" over the max, which is pierced primer territory. The Lantac pin in the Rexus and the JP comes up .0065" to .0085" short, and that's how you get light strikes. The standard pin in the JP is dead center. The other two passes, the standard pin in the Rexus and the Lantac pin in the Craddock, only clear the .028" minimum by .0025".

ASSUMING your bolt and firing pin lengths are similar to those used in this test, this means that:

• you should always use a 6.5 Grendel firing pin in a Craddock Precision HD ARC bolt
• you should never use a standard AR-15 firing pin in a Craddock Precision HD ARC bolt
• you should always use a standard AR-15 firing pin in a Rexus or JP Enterprises 6.5 Grendel bolt
• you should never use a 6.5 Grendel firing pin in a Rexus or JP Enterprises 6.5 Grendel bolt

About That Assumption

It's a big one. We measured one bolt from each brand, and bolt makers change suppliers and dimensions without telling anybody. Owners on the 6.5 Grendel forum measured Toolcraft's early Grendel bolts at about 2.800" overall and later ones at about 2.810". Same name on the box, .010" difference in the tail. Some short-tail bolts have even shipped with full-length pins.

There's no standard "6.5 Grendel firing pin" either. It's whatever the company selling it says it is. Some pins sold for Grendel bolts are about .010" short like our Lantac, and some measure as long as a standard pin or longer. Lantac lists ours for 6 ARC, 22 ARC, 338 ARC, and Grendel Type II bolts without giving a length, and its own 6 ARC carrier uses what it calls a "special Bolt & Firing Pin."

The bolt listings are hit or miss. The Primary Arms listing for the JP bolt says it uses a standard firing pin, and Rexus ships its Grendel carrier with a mil-spec pin, which lines up with what we measured. Craddock's listing says the HD ARC is compatible with mil-spec bolt components, but it doesn't say anything about firing pins, and our standard pin ran .0410" in it.

Here are the same rules, sorted by bolt instead of by brand:

• Short-tail Grendel bolt (about 2.800" overall with a .136" bolt face, which is what our Craddock acts like): use a short pin like the Lantac. A standard pin will stick out about .010" farther than it should.
• Long-tail Grendel bolt (about 2.810" overall with a .136" bolt face, the Alexander Arms pattern, which is how our Rexus and JP behave): use a standard pin. A short pin will come up about .010" shy.
• Any bolt with any pin: measure the protrusion before it goes to the range.

What Happens When It's Wrong

Too much protrusion can pierce primers. The tip punches through the primer cup, and the gas that comes back out through the hole erodes the tip of the firing pin and, given enough rounds, the firing pin hole in the bolt face. A gas-cut tip pierces primers more easily than a smooth one, so the problem feeds itself. If a pin has been piercing primers, plan on replacing it, not just re-gauging it.

Too little protrusion can give you light strikes: a shallow dent in the primer and no bang. Sometimes you get a late bang instead (a hangfire), which is why you keep the muzzle pointed somewhere safe for a full minute before you open the action on a dud. Harder primers show it first, so a marginal pin can run one box of ammo fine and start dudding on the next.

Neither symptom proves it's the pin. Hot loads and thin primer cups will pierce primers too, and so will a worn firing pin hole. And a weak hammer spring, some reduced-power triggers, or a firing pin channel packed with oil and carbon can give you light strikes with a perfectly good pin. Measure before you start throwing parts at it.

The Dimensions That Actually Decide It

If you're still reading.....

maybe it's because you noticed that the measured difference in the lengths of our firing pins was .008 inches but there was always about a .011 inch difference in protrusion. No, that's not an error. Protrusion is dictated by the distance from the front of the firing pin flange to the tip of the pin, not the overall length.

On a typical AR-15 bolt, Grendel or 5.56, the firing pin can only go forward until the front of its flange hits the back of the bolt tail. That's the stop. So two dimensions decide how far the tip sticks out:

• A, the firing pin: the front face of the flange to the tip.
• B, the bolt: the back of the bolt tail to the bolt face. That's the floor of the recess the case head sits in, not the front edge of the bolt.

Protrusion is A minus B. Every other number you'll see thrown around, including overall pin length and overall bolt length, only matters as far as it changes A or B.

Overall pin length is a decent stand-in for A when two pins come off the same drawing. It falls apart when they don't. Our Lantac pin is .008" shorter than our standard pin overall, but the protrusion numbers say its flange-to-tip is about .011" shorter. The extra .003" is somewhere behind the front of the flange (a thicker flange, a longer back end, or both), where it can't change protrusion at all.

Bolt length works the same way. B is the overall bolt length minus the bolt face depth:

• 5.56 bolt: 2.800" - .125" = 2.675"
• Long-tail Grendel bolt (Alexander Arms pattern): 2.810" - .136" = 2.674"
• Short-tail Grendel bolt: 2.800" - .136" = 2.664"

The long-tail Grendel bolt lands within .001" of a 5.56 bolt, so a standard pin protrudes about the same in both. The short-tail bolt is .010" shorter from tail to face than the long-tail one (.011" shorter than a 5.56 bolt), so the same pin sticks out about .010" farther. Our standard pin went from .0305" in the Rexus and .0325" in the JP to .0410" in the Craddock, a jump of .0085" to .0105". We didn't measure the Craddock's length, but that's about what a short-tail bolt should do.

Work it backward and you get the number nobody prints on a firing pin package. To land in the middle of the window (.032") in a bolt that's 2.675" from tail to face, the front of the flange has to sit about 2.707" from the tip. Take .010" off that and you've got a pin for a short-tail Grendel bolt.

Caliper measurements of Grendel bolts posted on the 6.5 Grendel forum work out to anywhere from about 2.660" to 2.675" from tail to face, depending on the brand and the sample. That spread is wider than the entire protrusion window. Standard-length pins measured in the same threads run from 3.273" to 3.281", and a Toolcraft pin sold for its 6.5 Grendel bolt measured 3.2825".

You'll also see people add bolt length and pin length together and compare the total to a magic number. That's backwards. A bolt that's longer from tail to face needs a longer pin, not a shorter one, so if anything it's the difference between those two numbers that should stay the same. And even that stops working the moment two pins put their flanges in different places, like ours do.

How to Check Yours

The military check is about as simple as it gets. Pull the bolt out of the carrier, drop the firing pin in from the back, push it all the way forward so the flange sits on the tail, and put the gauge on the bolt face. The issued gauge is a flat bar with a notch at each end. The pin should touch the gauge at the minimum end and clear it at the maximum end.

Brownells sells an AR-15/M16 protrusion gauge that works the same way, and Creedmoor makes a plunger-style gauge that you measure before and after. A dial indicator works too, if you've got one. If all you've got is calipers, take several readings and average them, because depth readings can wander by a couple thousandths.

Two Grendel-specific tips. Measure from the floor of the bolt face recess, where the primer sits, not from the front edge of the bolt. And if you want to know which kind of bolt you have before you order a pin, measure the overall length from the back of the tail to the front of the bolt, measure the bolt face depth, and subtract. Around 2.674" means a standard pin. Around 2.664" means a short one. Then check protrusion anyway.

If it's out of spec, the easy fix is the right pin. If you grind a long pin down instead, keep the tip smooth and round, because a flat spot or a sharp edge can pierce primers all by itself.

Everything here applies to 6 ARC, 22 ARC, and 338 ARC too. They all run the same .136" Type II bolt face, which is why Craddock sells the HD ARC for the whole family.

Bottom Line

There are long-tail and short-tail 6.5 Grendel bolts out there, about .010" apart, and each one needs its own pin. Mix them up and you're asking for pierced primers in one direction and light strikes in the other. Gauge every bolt and pin combination before it sees live ammo, and if a bolt shipped with a short pin, don't let the two get separated. They're a matched set.

• Protrusion window: .028" to .036"
• Protrusion: firing pin flange-to-tip minus bolt tail-to-face
• Bolt tail-to-face: bolt overall length minus bolt face depth
• 5.56 bolt: about 2.800" overall, .125" face, 2.675" tail-to-face
• Long-tail Grendel bolt: about 2.810" overall, .136" face, 2.674" tail-to-face, standard pin
• Short-tail Grendel bolt: about 2.800" overall, .136" face, 2.664" tail-to-face, short pin
• Flange-to-tip for .032" in a 5.56 bolt: about 2.707"